cmd/go, cmd/link: build c-archive as position independent on ELF

This permits people to use -buildmode=c-archive to produce an archive
file that can be included in a PIE or shared library.

Change-Id: Ie340ee2f08bcff4f6fd1415f7d96d51ee3a7c9a1
Reviewed-on: https://go-review.googlesource.com/24180
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/cgo/testcarchive/carchive_test.go

@@ -6,6 +6,7 @@ package carchive_test
 
 import (
 	"bufio"
+	"debug/elf"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -84,8 +85,13 @@ func init() {
 		cc = append(cc, []string{"-framework", "CoreFoundation", "-framework", "Foundation"}...)
 	}
 	libgodir = GOOS + "_" + GOARCH
-	if GOOS == "darwin" && (GOARCH == "arm" || GOARCH == "arm64") {
-		libgodir = GOOS + "_" + GOARCH + "_shared"
+	switch GOOS {
+	case "darwin":
+		if GOARCH == "arm" || GOARCH == "arm64" {
+			libgodir += "_shared"
+		}
+	case "dragonfly", "freebsd", "linux", "netbsd", "openbsd", "solaris":
+		libgodir += "_shared"
 	}
 	cc = append(cc, "-I", filepath.Join("pkg", libgodir))
 
@@ -487,3 +493,71 @@ func TestExtar(t *testing.T) {
 		}
 	}
 }
+
+func TestPIE(t *testing.T) {
+	switch GOOS {
+	case "windows", "darwin", "plan9":
+		t.Skipf("skipping PIE test on %s", GOOS)
+	}
+
+	defer func() {
+		os.Remove("testp" + exeSuffix)
+		os.RemoveAll("pkg")
+	}()
+
+	cmd := exec.Command("go", "install", "-buildmode=c-archive", "libgo")
+	cmd.Env = gopathEnv
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Logf("%s", out)
+		t.Fatal(err)
+	}
+
+	ccArgs := append(cc, "-fPIE", "-pie", "-o", "testp"+exeSuffix, "main.c", "main_unix.c", filepath.Join("pkg", libgodir, "libgo.a"))
+	if out, err := exec.Command(ccArgs[0], ccArgs[1:]...).CombinedOutput(); err != nil {
+		t.Logf("%s", out)
+		t.Fatal(err)
+	}
+
+	binArgs := append(bin, "arg1", "arg2")
+	if out, err := exec.Command(binArgs[0], binArgs[1:]...).CombinedOutput(); err != nil {
+		t.Logf("%s", out)
+		t.Fatal(err)
+	}
+
+	f, err := elf.Open("testp" + exeSuffix)
+	if err != nil {
+		t.Fatal("elf.Open failed: ", err)
+	}
+	defer f.Close()
+	if hasDynTag(t, f, elf.DT_TEXTREL) {
+		t.Errorf("%s has DT_TEXTREL flag", "testp"+exeSuffix)
+	}
+}
+
+func hasDynTag(t *testing.T, f *elf.File, tag elf.DynTag) bool {
+	ds := f.SectionByType(elf.SHT_DYNAMIC)
+	if ds == nil {
+		t.Error("no SHT_DYNAMIC section")
+		return false
+	}
+	d, err := ds.Data()
+	if err != nil {
+		t.Errorf("can't read SHT_DYNAMIC contents: %v", err)
+		return false
+	}
+	for len(d) > 0 {
+		var t elf.DynTag
+		switch f.Class {
+		case elf.ELFCLASS32:
+			t = elf.DynTag(f.ByteOrder.Uint32(d[:4]))
+			d = d[8:]
+		case elf.ELFCLASS64:
+			t = elf.DynTag(f.ByteOrder.Uint64(d[:8]))
+			d = d[16:]
+		}
+		if t == tag {
+			return true
+		}
+	}
+	return false
+}

src/cmd/go/build.go

@@ -338,6 +338,13 @@ func buildModeInit() {
 		case "darwin/arm", "darwin/arm64":
 			codegenArg = "-shared"
 		default:
+			switch goos {
+			case "dragonfly", "freebsd", "linux", "netbsd", "openbsd", "solaris":
+				// Use -shared so that the result is
+				// suitable for inclusion in a PIE or
+				// shared library.
+				codegenArg = "-shared"
+			}
 		}
 		exeSuffix = ".a"
 		ldBuildmode = "c-archive"

src/cmd/link/internal/ld/lib.go

@@ -163,7 +163,7 @@ func DynlinkingGo() bool {
 // relro.
 func UseRelro() bool {
 	switch Buildmode {
-	case BuildmodeCShared, BuildmodeShared, BuildmodePIE:
+	case BuildmodeCArchive, BuildmodeCShared, BuildmodeShared, BuildmodePIE:
 		return Iself
 	default:
 		return *FlagLinkshared
@@ -1642,7 +1642,7 @@ func stkcheck(ctxt *Link, up *Chain, depth int) int {
 		// onlyctxt.Diagnose the direct caller.
 		// TODO(mwhudson): actually think about this.
 		if depth == 1 && s.Type != obj.SXREF && !DynlinkingGo() &&
-			Buildmode != BuildmodePIE && Buildmode != BuildmodeCShared {
+			Buildmode != BuildmodeCArchive && Buildmode != BuildmodePIE && Buildmode != BuildmodeCShared {
 			ctxt.Diag("call to external function %s", s.Name)
 		}
 		return -1

src/cmd/link/internal/ld/symtab.go

@@ -155,7 +155,7 @@ func putelfsym(ctxt *Link, x *Symbol, s string, t int, addr int64, size int64, v
 	if x.Type&obj.SHIDDEN != 0 {
 		other = STV_HIDDEN
 	}
-	if (Buildmode == BuildmodePIE || DynlinkingGo()) && SysArch.Family == sys.PPC64 && type_ == STT_FUNC && x.Name != "runtime.duffzero" && x.Name != "runtime.duffcopy" {
+	if (Buildmode == BuildmodeCArchive || Buildmode == BuildmodePIE || DynlinkingGo()) && SysArch.Family == sys.PPC64 && type_ == STT_FUNC && x.Name != "runtime.duffzero" && x.Name != "runtime.duffcopy" {
 		// On ppc64 the top three bits of the st_other field indicate how
 		// many instructions separate the global and local entry points. In
 		// our case it is two instructions, indicated by the value 3.
@@ -362,7 +362,7 @@ func (ctxt *Link) symtab() {
 	// pseudo-symbols to mark locations of type, string, and go string data.
 	var symtype *Symbol
 	var symtyperel *Symbol
-	if UseRelro() && (Buildmode == BuildmodeCShared || Buildmode == BuildmodePIE) {
+	if UseRelro() && (Buildmode == BuildmodeCArchive || Buildmode == BuildmodeCShared || Buildmode == BuildmodePIE) {
 		s = Linklookup(ctxt, "type.*", 0)
 
 		s.Type = obj.STYPE

src/cmd/link/internal/x86/asm.go

@@ -51,8 +51,19 @@ func addcall(ctxt *ld.Link, s *ld.Symbol, t *ld.Symbol) {
 }
 
 func gentext(ctxt *ld.Link) {
-	if !ld.DynlinkingGo() && ld.Buildmode != ld.BuildmodePIE && ld.Buildmode != ld.BuildmodeCShared {
-		return
+	if ld.DynlinkingGo() {
+		// We need get_pc_thunk.
+	} else {
+		switch ld.Buildmode {
+		case ld.BuildmodeCArchive:
+			if !ld.Iself {
+				return
+			}
+		case ld.BuildmodePIE, ld.BuildmodeCShared:
+			// We need get_pc_thunk.
+		default:
+			return
+		}
 	}
 
 	// Generate little thunks that load the PC of the next instruction into a register.

src/cmd/link/internal/x86/obj.go

@@ -85,7 +85,7 @@ func archinit(ctxt *ld.Link) {
 		ld.Linkmode = ld.LinkInternal
 	}
 
-	if ld.Buildmode == ld.BuildmodeCShared || ld.Buildmode == ld.BuildmodePIE || ld.DynlinkingGo() {
+	if (ld.Buildmode == ld.BuildmodeCArchive && ld.Iself) || ld.Buildmode == ld.BuildmodeCShared || ld.Buildmode == ld.BuildmodePIE || ld.DynlinkingGo() {
 		ld.Linkmode = ld.LinkExternal
 		got := ld.Linklookup(ctxt, "_GLOBAL_OFFSET_TABLE_", 0)
 		got.Type = obj.SDYNIMPORT